The present invention relates to a method for converting a feed including an oxygenate to a product including a light olefin.
Light olefins, defined herein as ethylene, propylene, butylene and mixtures thereof, serve as feeds for the production of numerous important chemicals and polymers. Typically, light olefins are produced by cracking petroleum feeds. Because of the limited supply of competitive petroleum feeds, the opportunities to produce low cost light olefins from petroleum feeds are limited. Efforts to develop light olefin production technologies based on alternative feeds have increased.
An important type of alternate feed for the production of light olefins is oxygenates, such as, for example, alcohols, particularly methanol and ethanol, dimethyl ether, methyl ethyl ether, diethyl ether, dimethyl carbonate, and methyl formate. Many of these oxygenates may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for light olefin production.
Because light olefins are desirable products, research has focused on methods for optimizing the yields of light olefins. Research has also focused on methods for reducing undesirable by-products, particularly light saturates such as methane and ethane, because it is expensive to separate light saturates from light olefins. For example, for ethylene recovery, a typical recovery train requires a cold box, a de-methanizer, a de-ethanizer, and an ethylene/ethane splitter.
The reaction which converts oxygenates to olefins is exothermic and contributes to an overall temperature increase in a conversion reactor apparatus. This temperature increase may cause a temperature differential in the reactor. A temperature differential results when the temperature at the portion of the reactor in which the reaction ends is higher than the temperature of the portion of the reactor in which the reaction begins. Because the product selectivities of an oxygenate to olefin conversion reaction are, at least in part, temperature dependent, the temperature increase across the reactor affects the product slate of the conversion reaction. As the temperature in the reactor increases, light saturate production increases. Various methods have been used to remove or manage the heat of reaction in order to maintain the temperature of the reaction zone in a desired range.
U.S. Pat. No. 4,071,573 to Owen et al. describes a method for effecting chemical reactions of aliphatic hetero compounds, such as alcohols, halides, mercaptans, sulfides, amines, ethers and carbonyl compounds, with a fluidized crystalline zeolite catalyst and regeneration of a portion of the catalyst used in the fluid catalyst operation. The method described in the Owen et al. patent employs a catalyst recycle to each of the catalyst contact zones. Additionally, a quench gas distributor is employed in the reactor to further control exothermic conditions in the reactor. The distribution of catalyst or quench gas to discrete reactor zones requires additional equipment and controls, both of which add to the cost and complexity of the reactor system.
Another method for maintaining temperature in the reactor is to conduct the conversion reaction at a gas superficial velocity of less than 1 meter per second. At gas superficial velocities less than about 1 meter per second, an oxygenate to olefin conversion reaction occurs at near constant temperature, i.e., isothermal conditions, due to a high degree of back mixing of both solid and gas phases in the reaction. However, as the gas superficial velocity approaches plug flow behavior, i.e. 1 m/s, the isothermal nature of the conversion reaction is lost due to a decrease in the amount of back mixing of solid and gas phases that occurs as the gas superficial velocity increases. As the solid and gas phases move through s the reactor, the temperature of the reactor increases. For example, U.S. Pat. No. 4,513,160 to Avidan describes a process for the conversion of alcohols and oxygenates to hydrocarbons in a turbulent fluid bed reactor. Avidan describes that, when using a ZSM-5 zeolite catalyst, the turbulent regime is obtained when the superficial fluid velocity is 0.5-7 feet per second (0.15-2.13 m/s). See column 7, lines 23-65.
Thus, a need exists in the art for a method useful for maintaining the desired reaction temperature, managing the heat of reaction, providing good yields of the desired product, and avoiding the production of undesirable by-products, such as light saturates and coke.
The present invention solves the current needs in the art by providing a method for converting a feed including an oxygenate to a product including a light olefin. The method of the present invention is conducted in a reactor apparatus. As used herein, the term xe2x80x9creactor apparatusxe2x80x9d refers to an apparatus which includes at least a place in which an oxygenate to olefin conversion reaction takes place. As further used herein, the term xe2x80x9creaction zonexe2x80x9d refers to the portion of a reactor apparatus in which the oxygenate to olefin conversion reaction takes place and is used synonymously with the term xe2x80x9creactor.xe2x80x9d Desirably, the reactor apparatus, includes a reaction zone, an inlet zone and a disengaging zone. The xe2x80x9cinlet zonexe2x80x9d is the portion of the reactor apparatus into which feed and catalyst are introduced. The xe2x80x9creaction zonexe2x80x9d is the portion of the reactor apparatus in which the feed is contacted with the catalyst under conditions effective to convert the oxygenate portion of the feed into a light olefin product. The xe2x80x9cdisengaging zonexe2x80x9d is the portion of the reactor apparatus in which the catalyst and any additional solids in the reactor are separated from the products. Typically, the reaction zone is positioned between the inlet zone and the disengaging zone.
One embodiment of the method of the present invention comprises the following steps: providing a feed including an oxygenate; contacting the feed in a reaction zone of a reactor zone of a reactor apparatus with a catalyst including a molecular sieve, the contacting taking place under conditions effective to convert the oxygenate to a product including a light olefin, the conditions including a gas superficial velocity of at least two meters per second at least one point in the reaction zone; and recirculating a first portion of the catalyst to recontact the feed.
Another embodiment of the present invention is also directed to a method for converting a feed containing an oxygenate to a light olefin. The method comprises the following steps: (a) providing a reactor apparatus having an inlet zone, a reaction zone and a disengaging zone, the reaction zone being positioned between the inlet zone and the disengaging zone; (b) feeding a feed including an oxygenate to the inlet zone; (c) contacting the feed in the reaction zone with a catalyst including a molecular sieve, the contacting taking place under conditions effective to convert the oxygenate to a product including a light olefin, the conditions including a gas superficial velocity of at least two meters per second at at least one point in the reaction zone; (d) separating the product from the catalyst in the disengaging zone; (e) recirculating a first portion of the catalyst from the disengaging zone to the inlet zone; and (f) repeating steps (b) to (e).
Yet another embodiment of the present invention is directed to a method for converting a feed including an oxygenate to a product including a light olefin through the use of a non-zeolitic molecular sieve catalyst. The method comprises the following steps: providing a feed including an oxygenate; and contacting the feed in a reaction zone of a reactor apparatus with a catalyst including a non-zeolitic molecular sieve, the contacting taking place under conditions effective to convert the oxygenate to a product including a light olefin, the conditions including a gas superficial velocity of at least one meter per second at at least one point in the reaction zone.